Abstract At this time, there appears to be no FDA-approved antidote and/or reliable protocol for treating acute hydrogen sulfide (H2S) poisoning. Emergency medicine bulletins/pamphlets issued by several state authorities suggest the use of cyanide antidote kits containing nitrite-thiosulfate, or cobalamin, but the basic science that would justify this approach is lacking. Moreover, there are conflicting anecdotal case reports attesting to both the success and failure of cyanide therapeutics, together with supplemental oxygen delivery, employed in situations where H2S was known or suspected to be the toxic agent. Certainly the toxicology of H2S shares features in common with that of cyanide; for instance, both toxins are highly efficient disruptors of mitochondrial electron-transport chain function, with approximately identical inhibition constants (KI) for cytochrome c oxidase. It follows that in developing potential therapies for treating acute H2S intoxication, initial efforts should be directed toward overcoming inhibition of cytochrome c oxidase and the associated rapid cardiopulmonary collapse. However, suspected victims of H2S intoxication reaching the clinic sometimes succumb to the poisoning hours after the exposure, indicating slower mechanisms of toxicity subsidiary to cytochrome c oxidase inhibition. Accordingly, our specific aims are: Aim 1: To determine whether sodium nitrite ameliorates H2S intoxication in mice through a mechanism involving displacement of bound HS- from the active site of cytochrome c oxidase by NO; Aim 2: To determine whether supplemental oxygen ameliorates H2S intoxication in mice when given both alone and in conjunction with sodium nitrite. A variety of biophysical methods and behavioral assessment will be applied to investigating these matters in an effort to develop a protocol for treating H2S poisoning on a rational mechanistic basis using sodium nitrite in combination with supplemental oxygen.